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Protein and Amino Acids Play an Important Role
in Equine Nutrition.
Building
Your Horse's Engine
By
Lori K. Warren, Ph.D., PAS. Photos by Summer Best.
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High-protein,
low-carbohydrate diets, such as the Atkins diet and the South
Beach diet, have been quite popular over the past few years.
Although most of us don't go to that extreme when formulating
our horse's diet, protein is often overfed or blamed for a
variety of physical and behavioral problems in horses.
This article will address this misunderstood nutrient, including
the role of proteins in the body and guidelines for meeting
your horse's protein requirement. Some of the myths associated
with protein are discussed in the sidebar "Myth Defying."
Links in a Chain
Proteins are made up of many individual amino acids that have
been linked together in a chain. The specific amino acids
included in the chain, the order in which they appear, and
the length of the chain all dictates the function of that
particular protein.
Protein can be found throughout the body and plays a role
in virtually all of the horse's vital processes. Proteins
form muscle, skin, hair, and bone, as well as cartilage, tendons,
and ligaments. Furthermore, as hormones, enzymes and neurotransmitters,
proteins play a role in the regulation of growth, sleep, appetite,
blood clotting, and blood pressure, just to name a few.
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Building
the Chain
Proteins are continuously being broken down and rebuilt in the
body. This constant turnover of protein requires a steady supply
of amino acids. While the horse has the ability to synthesize
some of the amino acids needed to make body proteins, those
amino acids it can't make are "essential" and must
be supplied in the diet.
Dietary protein ingested by the horse is broken up in the stomach
and small intestine by digestive enzymes and acids. The individual
amino acids are absorbed through the wall of the small intestine
and into the bloodstream. Amino acids are carried by the blood
to sites throughout the body where they are needed and are reassembled
into specific body proteins.
The Missing Link
In order for the horse's body to build the required protein,
amino acids must be available in the correct amounts. If a particular
amino acid is not present, the building of the protein chain
stops, and the partially completed chain cannot function as
a protein.
The protein ingested by the horse is not always made up of the
same amino acids needed to make body proteins. In other words,
some amino acids in the feed are provided in abundance, while
others are found in limited quantities. Those found in limited
quantities are referred to as "limiting amino acids"
because their absence will limit the formation of many proteins
in the horse's body.
Using the feed ingredients commonly fed to horses, lysine is
the amino acid most likely to be deficient. Therefore, lysine
is referred to as the first limiting amino acid. Another amino
acid that may be provided in limited supply in horse diets is
threonine, which is referred to as the second limiting amino
acid. If the diet does not provide adequate lysine (or threonine),
synthesis of many body proteins will stop, even if all the other
amino acids needed to make that protein are present.
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Foals, weanlings
and yearlings need more quality protein and lysine and other
amino acids because they are making more muscle, bone and cartilage
than an adult horse that already has these tissues in place. |
Supplying
the Links
A good quality protein source is a feed that provides a sufficient
amount of essential amino acids, particularly lysine. There
are large variations in the lysine levels of different feedstuffs.
Additionally, the lysine level of a feed ingredient is not directly
proportional to its protein level (see Table 1). Therefore,
just because a feed has a high protein content doesn't necessarily
mean it provides an abundance of lysine.
The highest quality protein sources are milk products; they
are also the most expensive, thus their use is primarily reserved
for foal feeds. Soybean meal and canola meal are also high in
protein and high in lysine. Soybean meal and canola meal are
often included as a source of protein in commercial grain mixes.
By comparison, cottonseed meal and linseed meal, two other common
protein sources used in horse feeds, have a high level of protein,
but less lysine than soybean meal and, thus aren't considered
to be as high in quality (see Table 1). Lysine can also be manufactured
inexpensively, so many feed companies add synthetic lysine to
improve the overall amino acid profile of a commercial feed.
The requirements of the specific horse also have a bearing on
our definition of protein quality. For example, what might be
a poor quality protein for a weanling, who has a very high requirement
for lysine, would be very acceptable for an adult, whose requirements
for lysine are much lower.
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How
Much Protein is Needed?
Because horses build body proteins with amino acids, they have
an amino acid requirement, not a protein requirement. Fortunately,
we do not have to balance the diet for each of the 24 amino
acids found in nature. Most of the time, if your horse's lysine
requirement is met, his requirements for the other essential
amino acids are met.
The greatest need for protein, particularly high quality protein,
is with foals, weanlings, and yearlings. Growing horses are
making more muscle, bone and cartilage; therefore, they need
more lysine and other amino acids than an adult horse which
already has these tissues in place. Similarly, lactating mares
and those in the last three months of gestation have a greater
need for amino acids due to their increased demand for tissue
and milk synthesis.
While performance horses have an increased rate of protein turnover
in the body compared to idle horses, their protein requirements
do not increase as dramatically as those for growing horses
or broodmares. Often, the extra dietary protein needed to replace
and repair tissues can be supplied when their overall feed intake
is automatically adjusted upwards to accommodate a greater caloric
intake.
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Performance
horses have an increased rate of protein turnover in the body
and therefore have a higher protein requirement than idle horses. |
Feeds are typically sold based on protein content, so it is
easy to understand why many people use the percentage of protein
in a feed to make judgments on feed quality and feed selection
for their horses. However, your horse actually requires a finite
amount of protein (ie, pounds or grams of protein) in their
diet every day, NOT a percentage. As a result, many feeding
options exist to fulfill your horse's protein requirement.
To illustrate this point, consider the following example: Say
you have a 1,100 pound gelding you ride 4 times a week, giving
him a protein requirement of about 2 pounds per day. You feed
him 14 pounds of bermudagrass hay (7% protein) and 8 pounds
of a 14% protein grain mix, which meets his requirement ((14
x 0.07) + (8 x 0.14) = 2). You could actually feed him half
that amount of grain, and even use a grain with a lower percent
protein if you fed a different hay. For example, 14 pounds of
a timothy/alfalfa mix (13% protein) and 4 pounds of a 10% protein
grain mix would also meet his protein requirement.
The above example also illustrates the importance that forage
plays in meeting your horse's protein requirement. Forage (hay
or pasture) will likely be a significant source of protein for
your horse, simply because forage typically makes up the greatest
percentage of the diet. For example, say a 1,200 pound horse
was eating 20 pounds of timothy hay with 8% protein and 5 pounds
of grain with 14% protein. The hay would provide 70% of the
total protein in the diet (1.6 pounds), whereas the grain only
provides 30% (0.7 pounds). And for adult horses, the better
quality of forage you feed, the less you will have to rely on
additional protein.
| Table
1. |
|
Feed |
%
Crude Protein |
%
Lysine |
Protein
: Lysine Ratio |
| Oats |
14 |
0.49 |
28.6
: 1 |
| Corn |
10.4 |
0.28 |
37.1
: 1 |
| Bermuda
Grass |
12 |
0.40 |
30.0
: 1 |
| Alfalfa
Hay |
17 |
0.90 |
18.9
: 1 |
| Soybean
Meal |
54 |
3.44 |
15.7
: 1 |
| Linseed
Meal |
38.5 |
1.28 |
30.1
: 1 |
| Cotton
Seed Meal |
45.4 |
1.85 |
24.5
: 1 |
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| Source:
NRC, Nutrient Requirements of Horses, 5th Ed. (1989) |
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Overfeeding
Protein
Although we may not intentionally put our horses on an equine
version of the Atkins or South Beach diets, many horses are
fed much more protein than they need. A diet high in protein
has been blamed for many things that are untrue (see Myth Defying
sidebar); however, there are still some consequences to overfeeding
protein.
There is a large amount of protein in the body, but the horse
has no way of storing extra amino acids they have no immediate
need for. Extra amino acids are converted into urea and ammonia
and excreted in the urine. This leads to increased water intake,
increased urination and a noticeably strong ammonia smell in
the stall. An increased level of ammonia inside the barn could
cause lung irritation for both you and your horse. A high protein
diet also leads to increased water requirements, which may have
the greatest impact on horses in training or those competing
in hot weather.
The extra protein can serve as an energy (calorie) source, but
metabolically, it's an expensive process. The breakdown of protein
produces three to six times the amount of body heat compared
to the breakdown of carbohydrates or fats. It also yields considerably
less energy. This may be the reason why the Atkins or South
Beach diets produce weight loss in some diet-devoted humans,
but for a performance horse, extra dietary protein could cause
overheating if they are worked in hot weather. Bottom line:
protein should be fed to build the engine, not to fuel it.
Finally, protein is one of the most expensive components of
a feed; therefore, overfeeding protein is wasting money. Furthermore,
the additional ammonia and urea excreted from horses on a high
protein diet, as well as the extra soiled bedding that has to
be removed, may have negative impacts on the environment if
the manure is not disposed of properly.
In conclusion, protein is an important component of the horse's
diet and plays a role in just about every process in the body.
When selecting feeds, match the feed with your forage and your
horse's protein needs to avoid wasteful overfeeding. Selecting
a grain mix or supplement simply on the percentage of protein
it contains can be misleading. |
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Myth
Defying |
There are more myths about protein than any other nutrient provided
in the horse's diet. The following are explanations that will
defy some of the more common protein myths.
MYTH: Too much protein causes a horse to become hot-headed
and hard to handle
A diet high in starch and sugar has been shown to make a horse
more easily agitated, but there is no evidence that excess protein
has the same effect. The oats and corn in a feed mix are predominantly
starch (~70%). In contrast, most forages contain less than 15%
starch. An exception would be lush spring pasture (or lush winter
annual pasture). Lush, immature forage does have a high protein
content, but it also has a higher sugar content compared to
forages that are more mature. In fact, the amount of sugar consumed
in a day's worth of grazing a lush pasture is equivalent to
the sugar provided in about 8 pounds of sweet feed. Therefore,
a horse consuming a high grain diet or grazing a lush pasture
may experience a "sugar high" akin to a young trick-or-treater
on Halloween. Evidence for the link between hyper behavior and
high starch/sugar diets is also provided by observations that
replacement of a traditional grain mix with one that has added
fat or added fiber (low-starch mix) tends to suppress the high
spirited behavior, even though the protein content has not changed.
MYTH: A high protein diet increases the incidence of skeletal
problems such as osteochondrosis in young horses
Genetics, exercise and nutrition all play a role in the development
of healthy bones. As a result, the same factors are also linked
to the occurrence of developmental orthopedic disease in young
horses. Excessive growth and mineral imbalances have been correlated
with an increase in skeletal problems. Excessive protein was
blamed in the 1970's, but later studies disproved this connection.
Feeding a foal more protein than he needs does not increase
growth rate above that observed when foals are fed enough protein
to just meet their requirement. Unfortunately, it is still common
for many young horses to be kept on a low protein diet for fear
of causing developmental problems. Restricting protein intake
will certainly decrease growth rate, which may reduce the incidence
of skeletal problems. However, a low-protein diet can actually
be harmful to the foal by decreasing feed intake (and thus the
intake of other nutrients essential for growth) and retarding
skeletal development. Diets for the growing horse must supply
protein and minerals in proportion to the calorie content of
the diet; if you want to slow growth, do so by cutting calories
(from starch, sugar, fat), not by cutting back on protein.
MYTH: Excess protein damages the kidneys
The belief that excessive dietary protein causes kidney damage
probably originated with the observation that horses fed alfalfa,
which is high in protein, urinate more often. Excess protein
is converted into urea and ammonia and passed out of the body
in the urine. The more protein that is fed, the more urea and
ammonia there is to remove from the body; thus, the more urine
produced. It's a natural process that does not unnecessarily
stress the kidneys, as long as the horse has access to adequate
drinking water. However, a horse with compromised kidney function
(such as occurs in some older horses) should not be fed a high
protein diet since he will have a harder time filtering and
excreting the ammonia.
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Lori
K. Warren, PhD, PAS
Assistant Professor, Equine Nutrition
University of Florida, Institute of Food and Agricultural
Sciences
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